My 3.9 yo has a 101.3 fever suddenly with no other symptoms. should i be concerned? it seems weird.

Only sometimes: if the fever is >3 days without symptoms or he/she seems to have dehydration, change in mental status, poor color or shortness of breath. I would be more worried if she was sick prior to the fever starting. Viruses can cause fever and then rash after the 3 day period. That is why we often wait. If your doctor is customer friendly, they should see you when you wish, day one or day 3.

Probably not: Trivial viral infection is likely the cause. In case fever lasts more than three days, seeking medical attention would be appropriate.

Not uncommon: This is not uncommon at all. Many febrile illnesses start with a fever only, without other symptoms. If she/he is otherwise a healthy child, with no other medical problems, you can give ibuprofen or acetaminophen and fluids and observe. If the fever continues or she/he develops other symptoms, please take her/him to her/his doctor or be examined and properly diagnosed and treated.

Fever: In general fever is a sign of reaction of the body to infection. Infection could be of viral origin and can heal on its own without medications in most cases or could of bacterial origin and needs antibiotics. We can wait on asymptomatic fever up to a maximum of 5 days, but then a work up should be done to exclude infections or other diseases.